"Useful materials" is the generally accepted expression for those materials contained in garbage of any kind and any provenance which, on principle, can be reused.
The general trend is to exploit the garbage as perfectly as possible and to recover its useful materials as completely possible, thus obtaining secondary raw materials which can be recycled into the manufacturing process. Therefore, this will widely avoid the usual annihilation, or unused deposition, of the garbage and of the useful materials contained therein, as well as the environmental pollution connected to such behavior.
Industrially used recovering plants are to guarantee that the recovered fractions of useful materials are of the same species, and that the legal pollutant limits are met. A condition for this is that the recovering process is carried out in a plant which neatly separates the components, and does not allow any re-distribution between or enrichment of pollutant components in the fractions of the useful materials.
Thereby, an essential problem is controlling the whereabouts of the pollutants during the recovering process. For example, heavy metal compounds may appear as pollutants if they evaporate, or if removed by gasifying out of their original coherence with other inorganic compounds. And they may combine with the organic compounds present in the same mixture, wherefrom they can impeccably removed only with great difficulty. When such recovered organic working materials are reused, heavy metal compounds combined with them may get into the surroundings, where they have bad consequences, for example if they get into the food chain by using a secondary raw material as a fertilizer.
The danger of a splitting off possible pollutants by gasifying is particularly important if during a dehumidification process working temperatures of well above 100.degree. C. are used for dehumidification or for drying, as is often the case. As is generally known, some poisonous heavy metal compounds, in particular cadmium and mercury compounds, partially start evaporating already at about 115.degree. C.
Some of the known recovering plants for example those according to Swiss Patent No. 650,172, comprise a separate drying station for dehumidifying the useful materials, all fractions of the useful materials being jointly treated with dry air before their final separation and optional further crushing. Thereby, in most cases, high air temperatures are used in order to secure that the residence time of the material in the drying station be as short as possible. Moreover, this also effects a sterilization of the material. However, in said dehumidification process, the material is so highly heated that a re-distribution of the pollutants can hardly be avoided in practice.